Image pickup device



Jan. 3, 1961 s. GRAY IMAGE PICKUP DEVICE Filed Aug. 5, 1958 ff/.w /w/y/f INVENTOR. SIDNEY EBAY 2,957,255 Patented Jan. 3, 1961 IMAGE PICKUP DEVICE Sidney Gray, Somerville, NJ., assignor to Radio CorpoI ration of America, a corporation of Delaware Filed Aug. 5, 1958, Ser. No. 753,288v

S Claims. (Cl. 313-66) This invention is directed to an image device, and particularly to an improved image device of the photoemissive type.

One particular form of photoemissive image device is a pickup tube known as an image orthicon tube. The image orthicon tube, as presently known, includes an image section having a photoemissive cathode electrode upon which a scene to be televised is projected. Photoelectrons from the photocathode electrode are focused upon one surface of a semiconducting target electrode to establish a charge pattern on the target electrode corresponding to the optical image focused on the photocathode. The opposite surface of the target electrode is charged by electrical leakage through the target and is scanned by a low velocity electron beam from an electron gun to discharge the charge pattern on the target. By so doing, the reflected portion of the scanning beam is modulated by the charge pattern for producing a video television signal.

When light from a scene or object is picked up by an optical system and focused on the photocathode, photoelectrons are emitted from each of the elemental areas of the photocathode which are illuminated by the light in proportion to the intensity of the light striking the elements. The photoemitted electrons strike the target at ve- -locities between the irst and the second cross-over points of the secondary emission curve of the target and cause secondary electrons to be emitted by the target. The secondary electrons are collected by an adjacent mesh screen which is held at a xed positive potential with respect to the electron gun cathode. Emission of the secondary electrons produces a pattern of positive charges on the photocathode side of the target which corresponds with the pattern of light being received from the scene.

A broad dynamic range of operation for the image orthicon is desirable for color television pickup to provide optimum color reproduction. When scenes are viewed which have regions of low light intensities mixed with regions of extreme high light intensities at the same time, the low light intensity signals may at times be entirely suppressed. Suppression of low intensity signals occurs when extreme high light intensity signals cause secondary electrons to be emitted by the target in such numbers that some of the secondary electrons are not collected by the collector mesh. The secondary electrons travel freely some distance from their point of origin and are redistributed on other elemental areas of the target. The random redistribution of the seco-ndary electrons may cause contamination of adjacent charged areas thus distorting the low level components of the reproduced color picture.

An object of this invention is to provide an improved image device.

Another object of the invention is to provide an improved image orthicon tube which operates in a brood range of light intensities at any given time, and one which is capable of reproducing low light signals in the presence of high light intensities.

Another object is to provide a color image pickup tube operable in a large dynamic range which affords better color representation.

According to this invention, an apertured control electrode or screen is positioned adjacent to a photoemissive element of an image section of an image device, such as a photocathode of an image orthicon pickup tube, to limit the number of photoelectrons that passes from each elemental portion of the photoemissive element through the control electrode to a target electrode by an amount which depends on the magnitude of the light energy directed onto the elemental portion. The control electrode serves to broaden the operating range of the image device by limiting the number of photoelectrons available to provide secondary electron emission from the target electrode.

The invention is described in greater detail with respect to the drawings, in which:

Fig. 1 is a longitudinal cross section of a pickup tube incorporating this invention;

Fig. 2 is a graph which illustrates the light transfer characteristic curves of a conventional image orthicon tube and of an image orthicon tube incorporating the invention; and

Fig. 3 is an enlarged sectional View of part of the image section of an image orthicon tube incorporating this invention.

Similar reference numerals are used throughout the several views to denote similar elements.

Fig. l is a sectional view of an image orthicon tube comprising an envelope lil having an enlarged portion 12 at one end for enclosing an image section. Within the opposite end of the tubular envelope lli is an electron gun structure l@ comprising conventional heater, cathode and control grid structures, for producing an electron beam i6. An additional electrode 18 is formed as a wall coating on the inner surface of the tube envelope i0 for accelerating the electron beam lo toward a target electrode Ztl. Pairs of horizontal and vertical deflecting coils are formed into a yoke structure 21 surrounding the tube envelope lll, The deflecting coils 2l provide magnetic fields perpendicular to each other and to the tube axis. The deilecting coils 21 are connected, as is well known, to saw-tooth current sources (not shown) for providing frame and line scansion of the electron beam 16 over the surface of the target Ztl.

A decelerating electrode 22, formed as a ring, is mounted within the envelope immediately in front of target electrode 2i) and on the scanned side thereof. The decelerating electrode 22 provides uniform landing of the electrons of electron beam 16 ovei the target surface. Surrounding the tube envelope it) is a single coil 24 for providing a magnetic eld having lines of force parallel t0 the axis and extending from the gun structure 14 beyond the end of the envelope portion l2. The combined action of the magnetic iield of coil 24 and the electric elds provided by the electron gun i4, the coating electrode 18 and the decelerating electrode 22 provides a focusing action on the electrons of beam 16 to bring them to a small, Well defined point of focus on the surface of target 20.

Within the enlarged end of the tube envelope, there is a photocathode electrode 26 which is formed as la semitransparent layer of photoemissive material such as silver-antimony alloy sensitized with cesium. A pair of electrodes 23 and 30 are mounted coaxial to the tube envelope between the target 20 and the photocathode 26. These electrodes 28 and 30 provide an electric field which, in combination with the magnetic eld provided by coil 24, focuses the photoelectrons from photocathode 26 onto the target 20.

Target 20 is formed of a semiconductor such as a thin lm of glass having a slight electrical conductivity. Photoelectrons from photocathode 26 strike the surface of the glass target 20 at high velocity and release secondary electrons therefrom with an emission ratio greater than unity.

A tine mesh screen 34 is closely spaced from the photocathode side of the target 20. Screen 34 serves as a collector electrode and limits the potential on the photocathode side of the glass target surface 2i) to a potential approximating that of the screen 34. The glass target 20 is mounted within a short tubular ring-like mounting member 36, to which the metal mesh screen 34 is also attached, intermediate the ends of ring 36. As indicated in Fig. l, mounting ring 36 is in turn xed within a restricted or flanged portion of accelerating electrode 30. The potential of electrode 30 and mesh 34 is maintained at several volts positive relative to the potential of the cathode of electron gun 14.

During conventional operation of the tube of Fig. l, with no illumination on photocathode 26, electron beam 16 is scanned across the target 20 so that the scanned side of the target 20 is brought to substantially zero or gun cathode potential. When a light pattern from a viewed scene is focused on the photocathode 26, photoelectrons are emitted from each illuminated portion of the photocathode in an amount proportional to the light intensity thereon. The photoelectrons strike the surface of insulator glass target 20 and initiate a secondary emission from the bombarded areas to drive them positive toward the potential of collector screen 34. In this manner, there is set up on the photocathode side of the glass target 20 a charge pattern corresponding to the pattern of light or illumination focused upon the photocathode 26. Due to the extreme thinness of the glass target 20, and due to the slight electrical conductivity of the glass target, there is established a potential pattern on the scanned side of target 20 corresponding to the charge pattern on the photocathode side of the target. Accordingly, the potential of the scanned surface of the target 20 is varied from point to point, from substantially zero volts up to the potential of the collector screen 34.

The electron beam 16 approaches target 20 at very low velocity immediately in front of the target surface. When the beam 16 approaches the areas of the target which are at zero potential, it is reflected back toward the electron gun 14. However, more positive areas of the target surface will cause electrons from the approaching beam 16 to land on the target 20 in numbers sufficient to neutralize the positive potential produced by the photoelectrons on the opposite surface of the target 20. The negative charge deposited by the beam i6, and the posit1ve charge impressed by the photoelectrons neutralize each other in less than the time of one frame. The remaining electrons of the beam I6 are reflected back to the gun end of the tube. In this manner, as the electron beam 16 is scanned over the target 20, there is reflected toward the gun end of the tube a modulated return electron beam 38. The return beam 38 follows substantially the same path as the incident beam 16 and strikes a dynode electrode of a multiplier section 40 which 1s adiacent the gun structure 14. The modulated electron beam 38 is converted into an output video signal rnm a collector electrode 42 in the multiplier section In operation, the electron gun side of the target 20 is maintained, by way of example, at a potential of about zero volts with respect to ground by the scanning electron beam 16. The target collector mesh 34 is maintained at a positive potential of about two volts with respect to ground, for example. When an illuminated scene is viewed, the electron image from the photocathode 26 strikes the target 20 and causes secondary electrons to be emitted. With low illumination, nearly all the secondary electrons are collected by the collector mesh 34 since the potential of the target 20 does not ordinarily reach that of the mesh 34 during the 1/30 second storage time. As the illumination is increased, the potential of the glass target 20 rises to about that of the collector electrode 34 until a point is reached at which there is no field at the collector 34 to collect the secondary electrons. At and about this point, the secondary electrons are redistributed over the target 20 and land on other portions of the target.

In Fig. 2, signal output is plotted as a function of photocathode illumination. Curve (a) is a static light transfer characteristic of a conventional image orthicon tube wherein the target and collector mesh are close spaced. The signal output is linear until a point, called the knee of the curve, is reached where the target 20 reaches the potential of the collector 34. Above this point at the knee, the signal output is constant. When operating an image orthicon tube as presently known in the range of illumination O-A as shown, there is no problem of redistribution of secondary electrons at the target 20. However, after the knee of the signal output--illumination curve is reached, in the area of illumination A-B of Fig. 2, the secondary electrons emitted by the target 20 are no longer collected by the collector 34 but are redistributed over the target 20. This electron redistribution across the target 20 causes spurious image elements to be displayed, and also results in the dilution and suppression of color images thus providing an inaccurate color representation.

Fig. 2 shows the curve (c) for operation at higher levels of illumination in the range C-B. The knee of curve (c) at B is determined by the potentials of target 20 and collector mesh 34. Image elements of low level illumination are not registered as signals, whereas high lights are reproduced.

According to this invention, a control screen 44 is positioned closely adjacent to the photocathode 26, for example about 1 mm. away, facing the target 20, as shown in Figs 1 and 3. The control screen 44 comprises a flat fine-mesh electrode 46, preferably having about 750 holes per inch and formed from copper or nickel, for example. The mesh 46 is provided with a coating 48 of insulating material on the photocathode side, which may be formed thereon, for example, by evaporation of a fluoride, such as magnesium fluoride, calcium fluoride, or thorium oxyfluoride onto the mesh surface 46. A second coating of insulating material of fluoride may be evaporated over the insulator 48 in a vacuum to form a porous surface on the control screen 44. The porous surface of the screen 44 provides a low secondary emission characteristic so that the low velocity electrons from the photocathode 26 are not reected from the control screen 44. The control screen 44 is mounted within a tubular ring-like member 50, which is supported in a manner similar to the other electrodes.

With the control screen 44 biased at the same voltage as the photocathode 26, which may be 300 volts for example, some of the low velocity photoelectrons emitted from the photocathode 26 land on the control screen 44 due to their emission velocity. As the photoelectron current increases, the control screen 44 is driven in a negative direction with respect to the potential of the photocathode 26.

If the insulator surface 48 is charged 0.1 volt negative with respect to the photocathode 26, only those photoelectrons with 0.1 volt emission energy or greater can reach the mesh 44 or pass through the mesh. If the insulator surface potential is 0.2 volt negative with respect to the photocathode 26, photoelectrons with less than 0.2 volt emission energy cannot reach either the control mesh 44 or the target 20. Thus, from a conventional photoemission curve, the larger repelling charge on the control electrode for a high light level situation repels a larger percentage of the photoelectrons emitted from that area than the percentage of electrons repelled by a lower charged area. Fractional reduction of the number of electrons passing to the target 20 is increased as the photoelectron current is increased until there are no photoelectrons available with emission energy greater than the potential drop between the photocathode and the insulator surface. The illumination level at which the control electrode functions is adjusted by varying the D C. potential of the metal mesh 46 and thus the no-signal potential difference existing between the control electrode and the photocathode.

The bias of the control screen 414 thus elfectively limits the ilow of high light level photoelectrons from the elemental areas of the photocathode 26 having high light levels focused thereon. As the intensity level of the illumination on photocathode 26 increases, the biasing eiect of control screen 44 increases. At an extremely high level of illumination in the region B in Fig. 2 secondary electrons which are emitted from the target 20 may be redistributed. Thus, as indicated by the curve (b) of Fig. 2, the pickup tube of the invention is operable over an extended range from O to B of high and low level illumination without encountering the problem of random redistribution of secondary electrons.

The time rate of build-up and of decay of the control screen 44 is determined by choosing an insulating material 48 which provides the proper time constant and capacitance. The time constant (RC) of the insulator 48 is a function of the resistivity and the dielectric constant of the insulating material 48. The time constant, for example, may be set at 1/30 of a second, which is a viewing frame time under conventional television standards, thus allowing the bias of the contro-l screen 44 to be applied for approximately one frame time. The time constant characteristic of the insulator 48 provides for neutralization of the charge pattern on the insulator 43 in the time between successive scans by leakage of the charges to the conducting mesh electrode 46 which is maintained at a xed potential. Thus, a new pattern of charges representing the next scene is established on the insulator 48 prior to the reading out or scanning of the target electrode 20.

By means of this invention, photocurrents of the magnitude encountered in the image orthicon tube may be controlled provided that the control screen 44 has an adequate potential swing. The potential swing is dependent upon the resistance of the screen 44 which is a function of the resistivity, thickness, and area of the insulating material 48. In the image orthicon tube incorporating this invention, when operating with photocurrents 4having a current density of about l0-9 amperes per square centimeter, an insulating material 48 with a dielectric constant of 5 and a resistivity of 4 1012 ohmcm. for example, is used. The thickness of the insulating material 48 which is evaporated onto the conducting mesh 46 is approximately one micron. The thickness determines the potential swing across the insulator 4S of about .4 volt, which is the range of the control voltage.

Operation of an image orthicon tube with a control screen 44 biasing the current from photocathode 26 allows almost complete collection of the secondary electrons from the target Z@ for an extended range of illumination, and achieves a transfer characteristic which is readily expanded by conventional gamma-co-rrection means over a broad range of light intensities. Because redistribution of secondary electrons is eliminated in this range, low light signals may be obtained in the presence of high light signals. Color values are portrayed faithfully from a scene with a broad range of illumination.

A feature of this invention is that, in an image orthicon tube used for color transmission, colors may be reproduced in proper hue', and' portions of a scene do not change in hue or saturation according to the level of illumination of adjacent portions of a viewed scene.

This invention is generally applicable to any photoemissive image tube, or image section of a tube, which operates in a broad range of light intensities and which has a photocathode which causes secondary emission at an electrode.

What is claimed is:

1. An image orthicon comprising a photocathode and a target spaced from said photocathode, wherein photoelectrons are emitted from said photocathode according to light energy received from a scene and said photoelectrons are directed onto said target, and means including a con-trol grid element adjacent to said photocathode for controlling the current density of said photoelectrons arriving at said target in proportion to the number of photoelectrons from said photocathode.

2. An image orthicon comprising a photocathode and a target spaced from said photocathode wherein photoelectrons are emitted from said photocathode according to light energy received from a scene and said photoelectrons are directed to said target, and an apertured control element comprising a conducting mesh screen coated with an insulating material adjacent to said photocathode, and means for biasing said control element with respect to said photocathode so that photoelectrons emitted from elements of said photocathode increase said bias in accordance with an increase in light energy and said control element limits the percentage of said photoelectrons which are transmitted through said control element.

3. An image orthicon including a photocathode and a target spaced from said photocathode wherein photoelectrons are emitted from said photocathode according to light energy received from a scene, a control screen closely spaced adjacent to said photocathode, and means to apply a bias voltage to said control screen whereby the flow of photoelectrons from the photocathode to the target is limited when light of high level intensity is applied.

4. An image orthicon comprising an image section and a scanning section, said image section including a photocathode which emits photoelectrons upo-n the application of light energy, a target, a collector screen, means for directing pho-toelectrons which are emitted from said photocathode to said target, and a control grid element disposed adjacent to said photocathode for controlling the photoelectron ilow from said photocathode to said target, the amount of control exercised by said control grid being a function of said light energy.

5. An image section of a pickup device comprising a photoemissive element which emits photoelectrons in response to incident light, a target electrode upon which said photoelectrons are directed to cause emission of secondary electrons, a collector mesh to collect said secondary electrons, and a control grid electrode adjacent to said photocathode to control the photoelectron current from said photocathode to said target thus limiting said secondary emission the bias on said control grid being a function of said incident light.

6. An image section of an image orthicon tube comprising a photocathode for emitting photoelectrons in a pattern corresponding to a pattern of light energy received from a viewed scene, an insulator target electrode spaced from said photocathode to which said photoelectrons are directed at high velocities thereby causing emission of secondary electrons, a collector mesh disposed adjacent to said target electrode to collect said secondary electrons, and a control screen adjacent to said photocathode to reduce the number of photoelectrons which are transmitted to said target thereby allowing the reproduction of high level and low level illumination from the same scene.

7. An image section of an image orthicon tube as in claim 6, wherein said control screen comprises a ne wire mesh screen and an insulating material coated on one side of the mesh screen.

8. An image section of an image orthicon tube as in claim 7, wherein said insulating material is approximately 1 micron thick, and has a dielectric constant of about 5, and a resistivity of about 4 1O12 ohm-cm.

References Cited in the file of this patent UNITED STATES PATENTS Young Oct. 21, 1941 Cassman May 16, 1950 Raibourn July 18, 1950 Smith N0v. 18, 1958 

